Recently, under the circumstances of explosive diffusion of smartphones, portable tablet terminals, and the like, and the start of video distribution services, the increase in the volume of optical network transmission is in demand. As major components in optical communications to satisfy such needs, optical transceiver modules including fundamental functions such as electricity-light conversion, light-electricity conversion, amplification, reproduction demodulation are widely used. As the optical transceiver module, communication systems such as a wavelength division multiplexing system which transmits a divided signal to a plurality of channels via a plurality of different carrier waves and a parallel transmission system which transmits a plurality of channels as they are via a plurality of optical fibers (multi-core optical fiber tape) are used. In order to achieve these communication systems, a multi-channel optical transceiver module capable of inputting/outputting multi-channel electric signals in a single casing is essential, and its research and development have also been actively made recently.
The optical transceiver module includes a package composed of a casing accommodating a photoelectric transducer and the like therein, an electrical wiring connection unit constituting a part of the package, and an optical fiber connection unit. The package composed of the casing of the optical transceiver module is formed by ceramic, for example. The term “package” is often used to generally indicate a container to mount thereon and accommodate therein an electric element, an optical element, an integrated circuit (IC), and the like. However, in the below-described explanations associated with the present invention, a package having a broader form beyond a container which also includes the electrical wiring connection unit is to be used. For example, the package includes various forms such as a box-type package which is entirely hermetic sealed, a package not being hermetic sealed, a package having an open part on the top or the like so that an electric element can be seen, and a simple plate-like package having no side walls and only having an electric element or the like mounted thereon. Here, the package includes electrical wirings formed therein which are connected to an electric element or the like on the package, and an electrode pad, located in the vicinity of the end part of the package, which can connect to an external circuit as the electrical wiring connection unit.
The optical transceiver module has a function of converting an electric signal inputted from the external circuit via an electrical wiring into an optical signal to output it to an optical fiber, and in contrast, converting an optical signal inputted from the optical fiber into an electric signal to output it to the external circuit via the electrical wiring. To be more specific, a board constituting at least a part of an optical semiconductor device (such as a semiconductor laser, light receiving element, and modulation element) and a package on which the optical semiconductor device is mounted and an external circuit board in which a signal generation circuit (such as a drive circuit and an amplification circuit) is mounted are electrically connected by using a flexible printed circuits board (FPC) to realize a high-speed signal transmission.
For handling multi-channel signals in the optical transceiver module, electric terminals to input/output a plurality of electric signals by connecting them to the external circuit are required for an amount according to the number of multiplexes and the number of parallels for a system. Further, besides the terminals of electric signals, a number of electric terminals such as a terminal for power supply and a terminal for controlling an IC mounted inside the package and for monitoring the state of such control are required.
In the optical transceiver module which handles multi-channel signals and which is simultaneously required to be minimized, a flexible printed circuit board (hereinafter referred to as an FPC) which has higher density compared to connection using pins is used as electrical wirings as disclosed in PTL 1. The FPC is widely used in electric equipment and portable phones besides its implementation to the optical transceiver module. For example, in general LSI implementation, the FPC is used for bundling electrical wirings to other LSI circuits having multiple terminals and electric connections to a printed circuit board (PCB) and the like.
The FPC is used for electrical wirings between the PCB forming the external circuit and a ceramic-made package or any other board often used as an optical transceiver module casing. An electrode pad formed at the end part of the FPC is joined to an electrode pad on the PCB, the package, or the like by using highly reliable solder.
FIG. 9A and FIG. 9B are views illustrating a method of soldering an FPC using a conventional technique. FIG. 9A is a first view of the soldering method, which shows, in the end part of an FPC 200 before soldering and in an electrical wiring connection unit 100 in a package of an optical transceiver module, a cross section that vertically cuts each of the electrode pads with respect to each of the board faces. In the optical transceiver module, multiple electrode pads are formed at a part protruded, in a terrace shape, from one side face of a generally rectangular casing for soldering them to the electrode pads on the FPC. FIG. 9A depicts an electrical wiring connection unit in a terrace-like protruded portion 100 constituting a part of the package. It should be noted that a target to be joined to the FPC is not limited to the terrace-like protruded portion, but may also be an electrical wiring connection unit at its end part of the PCB as long as the optical transceiver module is constituted by a simple flat plate-like PCB. For simplifying descriptions including the case of the PCB, the side of the optical transceiver module is hereinafter referred to as a package 100. FIG. 9B is a second view of the soldering method, which indicates a state during a soldering process.
With reference to FIG. 9A, an electrode pad 101 is formed on the joining face side of the package 100. The FPC has any shape depending on a position of implementing a package, board, or the like to be connected and is composed of a thin and flexible material compared to normal printed circuit boards by forming, for example, a conductive foil on a basis of a film-like insulator. Multiple corresponding electrode pads to be soldered to the electrode pads 101 on the package 100 side are formed on at least one end of the FPC 200. As shown in FIG. 9A, each of the electrode pads at the end part of an FPC board 203 is formed on a position corresponding to conductive electrodes 202a, 202b sandwiching the board 203 therebetween which are connected via a via 201 filled with metal or the like. On the conductive electrode 202b on the side to be joined to the package 100, a solder layer 204 is formed in advance. As shown in FIG. 9B, when soldering the FPC 200 to the package 100 via the electrode pads, a method of soldering using a thermocompression tool 300, as disclosed in PTL 2, is used. When simultaneous heating and pressurizing are made, by the thermocompression tool 300, on the conductive electrode 202a which is a side opposite the joining face side of the FPC 200, heat is transferred to the joining face side via the via 201 to melt the solder layer 204, and thus the conductive electrode 202b and the electrode pad 101 are joined by soldering.